1. Field of the Invention
The invention relates to an electric power steering system that includes a ball screw device.
2. Description of the Related Art
There is an existing rack-assist-type electric power steering system (EPS) that includes a hollow shaft, through which a rack shaft passes, and that converts the rotation of the hollow shaft rotated by a motor into axial movement of the rack shaft using a ball screw device to supply assist force to a steering system.
In the EPS described above, the ball screw device is formed by arranging a plurality of balls between a ball screw nut secured to the hollow shaft and a threaded portion formed in the rack shaft. When a threaded portion formed in the inner periphery of the ball screw nut and the threaded portion formed in the outer periphery of the rack shaft face each other, a helical ball-rolling path is formed. Each ball arranged in the ball-rolling path rolls under load. Thus, the rotation of the ball screw nut is converted into axial movement of the rack shaft.
For example, Japanese Patent Application Publication No. 2006-256414 (JP-A-2006-256414) describes a structure for securing a ball screw nut to a hollow shaft. According to JP-A-2006-256414, the ball screw nut is fitted into a housing recess, formed in the inner periphery of the hollow shaft, in the axial direction, and the axial position of the ball screw nut is determined by a locknut. Employment of this structure makes it possible to reduce the axial length.
However, employment of the above-described structure causes an increase in the diameter of a portion of the hollow shaft, at which the ball screw nut and the hollow shaft overlap each other. Further, clamping the ball screw nut using the locknut may cause deformation of the ball screw nut, resulting in distortion of a thread groove that forms a ball-rolling path. The distortion hinders smooth rolling of each ball in the ball-rolling path, which may cause abnormal sound, vibration, and deterioration of steering feel.
Therefore, for example, Japanese Patent Application Publication No. 6-255501 (JP-A-6-255501) describes a structure in which flanges that extend radially outward are formed at axial ends of a ball screw nut and a hollow shaft, and the flanges are fastened to each other to connect the ball screw nut and the hollow shaft to each other. This structure makes it possible to secure the ball screw nut and the hollow shaft to each other in such a manner that the ball screw nut and the hollow shaft are not allowed to rotate relative to each other, without causing deformation of the ball screw nut.
However, in the above-described structure, the flanges need to be fastened to each other with a plurality of bolts. Fastening the flanges to each other with a plurality of bolts is cumbersome. Therefore, it is difficult to increase the productivity.
Japanese Patent Application Publication No. 2007-239782 (JP-A-2007-239782) describes a structure in which a threaded portion is formed in the outer periphery of a ball screw nut and the ball screw nut is screwed to the inner periphery of a hollow shaft. Employment of this structure makes it possible to disperse stress applied to the ball screw nut to the entirety of the outer periphery of the ball screw nut. Thus, it is possible to restrict deformation of the ball screw nut and distortion of a ball-rolling path due to the deformation. In addition, it is possible to improve the work efficiency during production.
However, it is difficult to form a threaded portion in the entirety of the outer periphery of a ball screw nut that has high strength and rigidity. In addition, providing the ball screw nut in a hollow shaft causes upsizing of a portion of the hollow shaft at which the ball screw nut is located. Therefore, a ball screw nut may be screwed (connected) to an axial end portion of a hollow shaft. Thus, it is possible to easily form a threaded portion in the ball screw nut without upsizing of a portion of the hollow shaft, to which the ball screw nut is connected.
However, even if the structure in which the ball screw nut is screwed to the axial end portion of the hollow shaft is employed, there is still a problem that should be solved.
In many coaxial-motor-type electric power steering systems such as an EPS described in Japanese Patent Application Publication No. 2007-239782 (JP-A-2007-239782) in which a hollow shaft serves also as a motor shaft and therefore a rack shaft and a motor are arranged coaxially with each other, openings formed at respective axial ends of a motor housing are blocked by side housings that are fitted to the respective axial ends. Because the motor shaft is rotatably supported in a motor stator formed on the inner periphery of the motor housing, the motor shaft is arranged coaxially with the rack shaft that passes through the housings in the axial direction.
In one of the two side housings, usually, a rack guide that constitutes a rack-and-pinion mechanism is formed. In the EPS described in JP-A-2007-239782, a ball bearing is provided in the side housing that blocks one of the openings and the motor shaft is supported by the ball bearing. Thus, the efficiency of assembly is improved, Therefore, one of the openings at the respective ends of the motor housing is blocked by the side housing in which the rack guide is formed, before the rack shaft is passed through the housing.
That is, the ball screw nut is screwed to the rack shaft to form a ball screw device, and then inserted in the motor housing through the remaining opening. Then, the ball screw nut is connected to the motor shaft in the axial direction. This structure makes it difficult to screw the ball screw nut to the motor shaft when the ball screw nut is screwed to the axial end portion of the motor shaft as described above.
In the electric power steering systems as those described in JP-A-2006-256414 and JP-A-2007-239782, a locknut and a ball screw nut are screwed to the inner periphery of a motor shaft. In the structure described in these documents, if an engaged portion is formed in the axial end face of the motor shaft, it is possible to perform the work for screwing the locknut and the ball screw nut to the inner periphery of the motor shaft with the rotation of the motor shaft restricted by a jig.
However, in the structure in which the ball screw nut is screwed to the axial end portion of the motor shaft, it is not possible to form the above-described engaged portion in the axial end face to which the ball screw nut is screwed. Therefore, it is difficult of restrict the free rotation of the motor shaft. Accordingly, it is difficult to prevent the simultaneous rotation of the motor shaft and the ball screw nut in the same direction that is caused when the ball screw nut is screwed to the motor shaft, that is, it is difficult to prevent the motor shaft from rotating together with the ball screw nut. On this point, there is still room for improvement.